The immunogenicity of tumor-associated antigens (TAA) may be augmented and the host's tumor rejection mechanisms related by virus infection of and virus expression in tumor cells. These immunological effects of virus on TAA will be studied in recent H-2 congenic methylcholanthrene-induced (MCA) tumors of inbred mice utilizing multiple in vitro and in vivo assays to monitor anti-tumor and anti-viral activities. The study includes the following objectives: 1. To complete tests for and establish the presence in "legitimate" H-2 alloantisera of antibodies reactive with TAA related to the viral envelope antigens (VEA) of endogenous type-C murine leukemia viruses and to show how such "anomalous" antibodies can lead to erroneous H-2 typing of VEA positive tumors cells. 2. To test whether or not anti-VEA antibodies and anti-TAA antibodies detected by cytotoxicity, immunofluorescence, and radioimmune precipitation in syngeneic anti-MCA tumor antisera are identical. 3. To measure and compare, using syngeneic and allogeneic immunizations, the immunogenicity and specificity of TAA on H-2 congenic MCA tumor cells maintained both in tissue culture and in vivo. 4. To re-evaluate immunologically the relationship between H-2 antigens and TAA on H-2 congenic MCA tumor cells. 5. To measure the immunological effects of conventional (influenza, VSV, EMC) virus infection of tumor cells on H-2 and TAA expression, tumor-cell immunogenicity and stimulation of host anti-tumor effector mechanisms.